METHOD AND DEVICE FOR SYMMETRIZING A CLOCK SIGNAL

Abstract: The present invention relates to a method and device for symmetrizing a clock signal. The leading or trailing edge of each pulse of the clock signal (Fin) to be symmetrized is used for forming the first edge of the pulse of the corresponding symmetrized clock signal (Fout). The pulse ratio of the symmetrized clock signal is measured; and on the basis of the measured pulse ratio, the time constant circuit (R1, C1), determining the location of the second edge of the pulse of the symmetrized clock signal, is adjusted by means of a control loop (R6, C2, A) in such a manner that the second edge of the pulse of the symmetrized clock signal (Fout) settles in a desired position in the symmetrized clock signal.

Description: Method and device for symmetrizing a clock signal The present invention relates to a method and device for symmetrizing a clock signal. Various components such as microprocessors have strict quality requirements regarding the symmetry of the pulse ratio of the used clock signal, especially in operating near the upper frequency limit specified for the component. The symmetrization of a clock signal can be carried out automatically by using a regulator comprising a feed-back loop. In a known (e.g. U.S. Patent 4,959,557) device, a clock signal is applied as AC-coupled via a low pass filter to a buffer circuit, from the output of which a symmetrized clock signal is obtained. The regulation of the pulse ratio is implemented by feeding the output signal of the buffer circuit to an integrator, from which a DC voltage proportional to the pulse ratio of the signal is obtained. This voltage is compared with the desired set value voltage of the pulse ratio in an amplifier, which produces a control voltage on the basis of the measured and the set-value voltage. This control voltage is passed as a bias voltage to the same input of the buffer circuit where the clock signal to be symmetrized has also been passed. The trigger level, which the buffer circuit detects in the incoming clock signal, is controlled by the bias voltage . Since the leading and trailing edges of the incoming clock signal have been graded by the low pass filter, the pulse ratio of the output signal of the buffer circuit also changes as the bias voltage changes. The controller seeks to find such a trigger level from the incoming clock signal above and below which level the durations of the clock signal states are equally long. The pulse ratio of commercially available conventional crystal oscillators is normally promised to be about 40/60. If a better pulse ratio is desired, the price of the oscillator goes up considerably. If the pulse ratio is adjusted manually, the productional testing costs of the manufactured device increase. The disadvantages of the known device based on a feedback control loop are the following: - it restricts the pulse ratio of an incoming signal, whereby the device seeks to find such a trigger level from the incoming signal above and below which level the periods of the signal are equally long. If the pulse ratio of an incoming signal is sufficiently poor, this kind of trigger level will not be found. - the accuracy of adjustment is dependent on the pulse ratio of an incoming signal. - both edges of the clock signal produced by the device are in random phase with respect to the phases of the edges of an incoming clock signal, i.e. it is not possible to use the device in decentralized applications, in which the phase of either edge of an incoming clock signal has to be maintained. The object of the present invention is to provide a method and device for symmetrizing a clock signal without the above-mentioned restrictions and disadvantages. To achieve this, the method of the invention is characterized in that the leading or trailing edge of each pulse of the clock signal to be symmetrized is used for forming the first edge of the pulse of the corresponding symmetrized clock signal; the pulse ratio of the symmetrized clock signal is measured; and on the basis of the measured pulse ratio, the time constant circuit determining the location of the second edge of the pulse of the symmetrized clock signal is adjusted by means of a control loop in such a manner that the second edge of the pulse of the symmetrized clock signal settles in a desired position in the symmetrized clock signal. By means of the method according to the invention, it is possible to symmetrize even a clock signal with a poor pulse ratio so as to be suitable for use in applications of critical timing. The implementation of the device of the invention is simple. The restrictions of the solution based on a known controller have been obviated by forming one edge of the symmetrized clock signal directly from the clock signal to be symmetrized, and the other edge by means of an independently adjustable time constant circuit. The device of the invention and the other preferred embodiments of the invention are characterized by what is presented in the claims that will follow later. In the following, the invention will be described in more detail by means of an example with reference to the accompanying drawing, in which Figure 1 shows the device of the invention for symmetrizing a clock signal, and Figure 2 shows a timing diagram relating to the circuit according to Figure 1. In Figure 1, the incoming clock signal F in to be symmetrized is passed as a clock to a D-flip-flop circuit IC. A 1-state signal is obtained at the leading edge of the clock signal F in from the output Q of the IC. At the same moment, a 0-state signal is obtained from the output Q of the IC, whereby the voltage of a diode Dl changes to reverse. The signal R of the D-flip-flop IC is connected to a time constant circuit Rl and Cl. By means of a voltage V adJ , the voltage of a capacitor Cl is discharged until the IC detects the signal R. At this moment, the output Q of the D-flip-flop changes to state 0, and the output Q to state 1, whereby the capacitor Cl is charged through the diode Dl to a constant voltage, and the signal R is deactivated. The phases mentioned above are repeated after each leading edge of the incoming signal. A clock signal F out is thus obtained from the output of the D-flip-flop IC, the leading edge of this clock signal coinciding with the leading edge of the incoming clock signal F in to be symmetrized, and the position of the trailing edge of this clock signal being determined by the time constant circuit Rl and Cl. The formed F out signal is symmetrized by using a feedback control loop. The clock signal F out is applied to an integrator R6 and C2; and a DC voltage V d , which is proportional to the pulse ratio of the F out signal and which is obtained from the output of the integrator R6 and C2, is applied to the inverting input of an operational amplifier A. A voltage V ref corresponding to the set value of the pulse ratio is also applied to the amplifier A via a voltage divider R4 and R5. The amplifier A compares the voltage V d , which is proportional to the pulse ratio of the formed clock signal F out , with the set value voltage V ref , and forms a control voltage V adj , by which the time constant circuit Rl and Cl is controlled. If the length of the 1-state of the formed clock signal is too great with respect to the length of the 0-state, the amplifier A reduces the control voltage V adj . The capacitor Cl is thus discharged faster, and the signal R is activated earlier, whereby the pulse ratio is corrected. If the 0-state of the clock signal F out is too long, the amplifier A increases the control voltage V adJ , whereby the signal R is activated later. As a final result, a symmetrized clock signal F out is obtained from the D-flip-flop IC. The values of the time constant circuit Rl and Cl are dimensioned according to the frequency of the used clock signal. The accuracy of the dimensioning also affects the obtained accuracy of the adjustment. The device of the invention can be used for symmetrizing clock signals at digital telephone exchanges; for instance, a clock signal of the frequency of 16.384 MHz formed by the crystal oscillator of the synchronization unit of a Nokia DX 200 exchange. In the following, the part list of the device suitable for this purpose is presented with references according to Figure 1. Fι» = = 16.384 MHz, v ref = = 2. .5 V IC 74AC74 R5 1 kΩ A LF356 R6 1 kΩ Rl 270 Ω Cl 47 pF R2 10 kΩ C2 100 nF R3 1 kΩ Dl 1N4153 R4 10 kΩ All the components in the above part list are standard components, the acquisition and use of which are obvious to a professional in the art. Furthermore, it is apparent that the component values of the time constant circuit must be dimensioned application- specifically according to the frequency of the clock signal to be symmetrized. In this case, the gain of the amplifier A is selected to be -10. The amplifier compares the reference voltage V ref (+2.5 V) of the set pulse ratio with the voltage V d , which is proportional to the pulse ratio of the formed clock signal. The amplifier A seeks to adjust the voltage V d to the set value V rβf . It is apparent to a professional in the art that the different embodiments of the invention are not restricted to the example presented above, but that they can vary within the scope of the appended claims.

Claims: Claims 1. A method for symmetrizing a clock signal, c h a r a c t e r i z e d in that the leading or trailing edge of each pulse of the clock signal (F in ) to be symmetrized is used for forming the first edge of the pulse of the corresponding symmetrized clock signal (F out ); the pulse ratio of the symmetrized clock signal is measured; and on the basis of the measured pulse ratio, the time constant circuit (Rl, Cl) determining the location of the second edge of the pulse of the symmetrized clock signal is adjusted by means of a control loop (R6, C2, A) in such a manner that the second edge of the pulse of the symmetrized clock signal ( F out ) settles in a desired position in the symmetrized clock signal. 2. A method according to claim 1, c h a r ¬ a c t e r i z e d in that the voltage (V d ), which represents the measurement result of the pulse ratio of the clock signal (F out ) symmetrized in the control loop (R6, C2, A), is compared with a pre-set voltage (V ref ), and a control voltage (V adj ) is formed for the time constant circuit (Rl, Cl), the magnitude of this control voltage being dependent on the length of the 1-state of the formed clock signal (F out ) with respect to the length of its 0-state. 3. A device for symmetrizing a clock signal, c h a r a c t e r i z e d in that the device comprises means (IC) for forming the first edge of the pulse of the symmetrized clock signal (F out ) by means of the edge of each pulse of the corresponding clock signal (F in ) to be symmetrized; a control loop (R6, C2, A) for measuring the pulse ratio of the symmetrized clock signal and for forming the control signal (V adj ); and a time constant circuit (Rl, Cl) for setting the second edge of the pulse of the symmetrized clock signal (F out ) in a desired position in the symmetrized clock signal on the basis of the control signal. 4. A device according to claim 3, c h a r - a c t e r i z e d in that the amplifier (A) of the control loop (R6, C2, A) is arranged to compare the voltage (V d ) representing the measurement result of the pulse ratio of the symmetrized clock signal (F out ) with the pre-set voltage (V ref ), and to form a control voltage ( v adj) for "the time constant circuit (Rl, Cl), the magnitude of this control voltage being dependent on the length of the 1-state of the formed clock signal (F out ) with respect to the length of its 0-state.

Inventors: RUUSKANEN MARKKU, RUUSKANEN, MARKKU
Applicants: NOKIA TELECOMMUNICATIONS OY, RUUSKANEN MARKKU, RUUSKANEN, MARKKU
Application year: 1994
Main classification: H03K5
IPC codes: H03K5